1. Field of the Invention
The present invention relates to an image reading device and a radiation image pick-up apparatus, and specifically to an image reading device comprising a sensor substrate having at least a plurality of photosensors for executing photoelectric conversion of the light entered in the sensor substrate and to a radiation image pick-up apparatus utilizing such image reading device. More specifically, it relates to an image reading device formed with a large-area process to enable a large area image reading, and a radiation image pick-up apparatus utilizing such image reading device.
2. Related Background Art
For reading information such as an image for example in an image reader, there are conventionally known a reading device employing a reduction optical system and a CCD sensor, and a reading device employing a television camera.
Also, based on the recent development of a semiconductor material for photoelectric conversion by utilizing a non-single-crystalline semiconductor represented by hydrogenated amorphous silicon, there is recently proposed an image reading device having a plurality of photosensors formed on a large-sized substrate by using this material which can read an image with a large area.
Besides, the non-single-crystalline semiconductor can be utilized not only as the photoelectric conversion semiconductor material of the photosensor but also as the semiconductor material of the thin film transistor. Therefore, signal processing portions can also be formed on the large-sized substrate together with the photosensors. Furthermore, the photoelectric conversion semiconductor layer of the photosensor and the semiconductor layer of the thin film transistor can be composed of the same layer.
Furthermore, in case of forming a capacitance element such as a capacitor on the same substrate, a semiconductor layer may be interposed between the opposed electrodes if they are mutually insulated. Based on this fact, it is possible to make common the order of deposition for the films constituting the photosensor, the films constituting the thin film transistor and the films constituting the capacitance element, and the films constituting each element can therefore be made common.
It was therefore rendered possible to produce the image reading device with a large area at a low cost.
As another application of such large-area image reading device, there is also conceived a device for converting a radiation image into a visible image by a fluorescent plate which emits visible light by absorbing a radiation such as X-ray, and reading such visible image. A compact X-ray image pick-up apparatus of high performance is being realized by incorporating such image reading device thereto.
In Japanese Patent Application Laid-Open No. 9-298287 the present inventor has proposed one of the above-mentioned image reading devices.
FIG. 13A is a plan view of one pixel of the image reading device proposed by the present inventor, and FIG. 13B is a cross-sectional view taken along the line 13Bxe2x80x9413B in FIG. 13A. Also, FIG. 14 is an equivalent circuit diagram of one pixel of this image reading device.
One pixel is composed of a MIS photosensor S11, and a driving thin film transistor T11 serving as the driving portion of the photosensor. These drawings show a signal wiring SIG, a gate line gn of the driving thin film transistor T11, the upper electrode S and lower electrode G of the MIS sensor, and capacitances Cgs and Cgd respectively formed by overlapping the gate electrode with the source electrode and by overlapping the gate electrode with the drain electrode. Charges generated in the MIS photosensor S11 by light are accumulated in the capacitances Cgs and Cgd through the thin film transistor T11, and are then read by a reading circuit not shown in the drawings. Though the above is limited to one bit, but in the fact the capacitances Cgs and Cgd are the sum of capacitances connected to other transistors, respectively. Thus the accumulation capacitances utilize Cgs and Cgd.
FIGS. 15A and 15B are energy band illustrations Qf the photosensor showing the operations in a refreshing mode and a photoelectric conversion mode, respectively, wherein numerals 1 to 5 indicate each layer in the thickness direction thereof. In FIG. 15A showing the refreshing mode, the electrode S is given a negative potential with respect to the electrode G, whereby holes represented by black circles (xe2x97xaf) in a hydrogenated amorphous silicon layer 3 are guided to the electrode S by the electric field. At the same time, electrons represented by open circles (∘) are injected into the hydrogenated amorphous silicon layer 3. In this time, a part of the holes and a part of the electrons recombine and vanish in an N+ hydrogenated amorphous silicon layer 2 and the hydrogenated amorphous silicon layer 3. If this state continues for a sufficiently long time, the holes in the hydrogenated amorphous silicon layer 3 are removed from the hydrogenated amorphous silicon layer 3. When next state becomes a state of the photoelectric conversion mode shown in FIG. 15B, the electrode S is given a positive potential with respect to the electrode G, whereby the electrons in the hydrogenated amorphous silicon layer 3 are instantaneously guided to the electrode S. However, the holes are not guided to the hydrogenated amorphous silicon layer 3 because the N+ hydrogenated amorphous silicon layer 2 functions as an injection inhibiting layer. When light enters the hydrogenated amorphous silicon layer 3 in this state, the light is absorbed and electron-hole pairs are generated. The electrons are guided to the electrode by the electric field, while the holes move in the hydrogenated amorphous silicon layer 3 and reach the interface of a hydrogenated amorphous silicon nitride layer 4 but are stopped at the interface and remain in the hydrogenated amorphous silicon layer 3. As the electrons move to the electrode S while the holes move in the hydrogenated amorphous silicon layer 3 to the interface with the hydrogenated amorphous silicon nitride layer 4, a current flows from the electrode G in order to maintain the electrical neutrality in the device. This current corresponding. to the electron-hole pairs generated by the light is proportional to the incident light.
FIG. 16 shows the entire circuit of the image reading device. Photosensors and thin film transistors for driving the photosensors can be formed on the same substrate by the same process. In the circuit diagram there are shown photosensors S11 to S33, thin film transistors T11 to T33 for driving the photosensors, a reading power source Vs and a refreshing power source Vg connected, respectively through switches SWs and SWg, to the lower electrode G of all the photosensors S11 to S33. The switch SWs is connected through an inverter to a refreshing control circuit RF while the switch SWg is connected directly to the refreshing control circuit RF. The switch SWg is turned on during the refreshing period while the switch SWs is turned on during other periods. The output signal is supplied by a signal wiring SIG to a detecting integrated circuit IC.
In the circuit shown in FIG. 16, nine pixels are divided into three blocks, and the outputs of three pixels in each block are transferred simultaneously and signals are successively converted into outputs by the detecting integrated circuit. For the purpose of simplicity, there is illustrated a two-dimensional image input portion of nine pixels but there are in practice provided a larger number of pixels in a high density. As an example, in case an image reading device of 40xc3x9740 cm is formed with pixels of a size of 150xc3x97150 xcexcm, there are provided approximately 1.8 million pixels.
FIG. 17 schematically shows an X-ray image reading device formed by combining the above-described image reading device with a fluorescent plate, and an X-ray image pick-up apparatus utilizing such image reading. device.
X-ray emitted from an X-ray source 1701 hits an object 1702, and a part thereof enters an image reading device 1705 as the transmitted light, while another part enters the image reading device as the scattered X-ray. A grid 1703 removes the scattered X-ray from the object. Substantially only the transmitted light enters the fluorescent plate 1708, is absorbed therein and is converted into visible light depending on the intensity. This visible light enters the photosensors as an image of 1xc3x97 magnification, and is incorporated as an image. There are also shown a sensor substrate 1704, a case 1706 for the image reading unit, and an image reading unit 1707.
The problems of the conventional image reading device described above will be explained by referring to an image reading device for the X-ray image pick-up apparatus.
In use of the fluorescent plate mainly composed of Gd2O2S:Tl3+ having a high light emitting efficiency, when X-ray enters this fluorescent plate, the light emitted from the plate has a green light having a wavelength of xcex=550 nm with the largest intensity.
The light emitted from the fluorescent plate reaches the sensor substrate while it is scattered, is detected by the photosensors and is outputed as image signals to the outside.
However, in the conventional image reading device in which the photosensors and thin film transistors are generally formed on a transparent insulating substrate such as a glass substrate, the light entered in the interior of the substrate through the gap between the pixels is reflected on the back surface of the substrate because it is transparent, thereby generating stray light and degrading the sharpness of the image reading device.
More specifically, the back surface of the sensor substrate is in contact with the air in an extreme position, thereby showing a large difference in the refractive index. Generally, the light entering in an interface between layers having difference refractive indexes is reflected in a proportion corresponding to the difference in the refractive index. The reflective coefficient R of the reflected light in case of a normal reflection is represented by:
R=(n1xe2x88x92n2)/(n1+n2)
wherein n2 is the refractive index of air and n1 is the reflective index of the sensor substrate.
The reflection intensity is proportional to R2. In case the sensor substrate is composed of glass, n1=1.5, and, for n2=1.0, there is obtained R=0.2 and R2=0.04 so that 4% of light is reflected at the interface.
The actual system includes not only the normal reflection but also the light obliquely entering the interface, and the reflectance for such light becomes larger than the normal reflectance. Also under the condition of total reflection, the incident light is reflected by almost 100%.
Consequently the light entering the interior of the substrate from the surface thereof with an angle reaches the back surface of the substrate, on which at least a part of such light is reflected and is absorbed by the adjacent pixels to generate optical crosstalk, which causes degradation of the sharpness of the image reading device.
The first image reading device of the present invention comprises a sensor substrate having at least a plurality of photosensors for photoelectric conversion of the light entered in the sensor substrate, wherein the sensor substrate is composed of a material capable of absorbing the light entered in the sensor substrate.
The second image reading device of the present invention comprises a sensor substrate having at least a plurality of photosensors and a base member adhered to the sensor substrate via an adhesive layer, wherein the base member is composed of a material capable of absorbing the light entered from the sensor substrate.
The third image reading device of the present invention comprises a sensor substrate having at least a plurality of photosensors and a base member adhered to the sensor substrate via an adhesive layer, wherein the base member has a layer of a material capable of absorbing an entering light on the surface of the base member at the side of the sensor substrate.
The fourth image reading device of the present invention comprises a sensor substrate having at least a plurality of photosensors and a base member adhered to the sensor substrate via an adhesive layer, wherein the base member has a layer of a material capable of absorbing an entering light on the surface of the base member at the side opposite to the side of the sensor substrate.
The fifth image reading device of the present invention is the above-mentioned second image reading device, provided that the sensor substrate, the adhesive layer and the base member have substantially the same refractive index (including the same refractive index).
The sixth image reading device of the present invention is the above-mentioned third image reading device, provided that the sensor substrate, the adhesive layer and the layer of the material capable of absorbing the entering light have substantially the same refractive index (including the same refractive index).
The seventh image reading device of the present invention is the above-mentioned fourth image reading device, provided that the sensor substrate, the adhesive layer, the base member and the layer of the material capable of absorbing the entering light have substantially the same refractive index (including the refractive index).
The eighth image reading device of the present invention is any one of the above-mentioned first to seventh image reading devices, provided that:
the photosensor is formed by stacking a first electrode layer, a first insulating layer for inhibiting the passage of carriers of a first conductivity type and carriers of a second conductivity type different from the first conductivity type, a photoelectric conversion semiconductor layer, a second electrode layer, and an injection inhibiting layer for inhibiting injection of the carriers of the first conductivity type into the photoelectric conversion semiconductor layer, the injection inhibiting layer being provided between the second electrode layer and the photoelectric conversion semiconductor layer;
switching means to be respectively connected to the photosensors are formed on the sensor substrate together with the photosensors;
the switching means in a refreshing operation provide the photosensors with an electric field in a direction of guiding the carriers of the first conductivity type from the photoelectric conversion semiconductor layer to the second electrode layer, and the switching means in a photoelectric conversion operation provide the photosensors with an electric field in a direction of retaining the carriers of the first conductivity type generated by the light entering the photoelectric conversion semiconductor layer in the photoelectric conversion semiconductor layer and guiding the carriers of the second conductivity type to the second electrode layer, the switching means being controlled in the photoelectric conversion operation so as to detect, as optical signals, the carriers of the first conductivity type accumulated in the photoelectric conversion semiconductor layer or the carriers of the second conductivity type guided to the second electrode layer; and
the plurality of photosensors are arranged two-dimensionally and are divided into a plurality of blocks, and the switching means are operated for each block to detect the optical signals.
Also the radiation image pick-up apparatus of the present invention utilizes any one of the above-mentioned first to eighth image reading devices.